Let the plant tell you whether nitrates are a problem. Nitrates absorbed from the soil by plant roots are normally incorporated into plant tissue as amino acids, proteins and other nitrogenous compounds. Thus, the concentration of nitrate in the plant is usually low. The primary site for converting nitrates to these products is in growing green leaves. In grasses, nitrates accumulate at the base of the main shoot. If nitrates are accumulating then new growth (tillers) will likely be visible near the base of the plant.
Under stressful growing conditions, especially drought, this conversion process is slowed, causing nitrate to accumulate in the stalks, stems, and other conductive tissue. If moisture conditions improve, the conversion process accelerates and within a few days nitrate levels in the plant returns to normal. A return to non-stressed conditions following substantial rainfall should decrease nitrate accumulation, but chopping should be delayed for 3 to 5 days.
Check fields affected by drought that have plants stunted with significant firing. Nitrate toxicity will likely be a problem if growth is reduced to less than 50% of normal and/or high levels of nitrogen were applied.
Raising the bar
If you suspect an issue, then raise the cutter bar. The highest concentration of nitrates is in the lower part of the stalk or stem (Table 1), so raising the cutter bar on a corn silage chopper will leave most nitrates in the field. Depending upon farm forage needs, raising the cutter-bar on the silage chopper reduces yield but increases quality. For example, raising cutting height reduced yield by 15%, but improved quality so that Milk per acre of corn silage was only reduced 3-4% (Lauer, 1998). In addition the plant parts with highest nitrate concentrations remain in the field.
|Table 1. Nitrate nitrogen of corn plant parts harvested for silage.|
|Upper 1/3 of stalk||153|
|Middle 1/3 of stalk||803|
|Lower 1/3 of stalk||5524|
|derived from Hicks, Minnesota|
Nitrates are reduced through ensiling
Nitrate concentration usually decreases during silage fermentation by one-third to one-half, therefore sampling one or two weeks after filling will be more accurate than sampling during filling. If the plants contain nitrates, a brown cloud may develop around your silo. This cloud contains highly toxic gases and people and livestock should stay out of the area. The only way to know the actual composition of frosted corn silage is to have it tested by a good analysis lab.
It is prudent to follow precautions regarding dangers of nitrate toxicity to livestock (especially with grazing and green-chopping) and silo-gasses to humans when dealing with drought-stressed corn.
If drought-stressed corn is ensiled at the proper moisture content and other steps are followed to provide good quality silage, nitrate testing should not be necessary. It is always prudent to test or feed to a few cull cows.
Samples taken for nitrate test must be frozen or analyzed immediately as nitrate will decline in tissue over 3 to 4 hours. If above toxic, levels feed hay or some other forage in the morning and graze corn a couple hours in the afternoon.
Marshfield Plant and Soil Analysis Laboratory
8396 Yellowstone Dr.
Marshfield, WI 54449-8401
Phone: (715) 387-2523
Lauer, J. 1998. Corn Silage Yield and Quality Trade-Offs When Changing Cutting Height. Agronomy Advice. December, 1998. Field Crops 28.47-20.